The present invention relates to a hybrid welding process and to a hybrid welding set combining a laser beam and an electric arc, particularly a plasma arc, and to its application to the welding of pipes.
Laser technology has been known and widely used for years to weld various metallic materials, such as workpieces made of alloy steels or non-alloy steels, coated steels, stainless steels, aluminium and aluminium alloys, or to weld pipes made of various metals.
In general, a laser welding set for welding pipe comprises, apart from the means for feeding and holding the pipe, a solid-state or gas laser oscillator producing a coherent monochromatic high-energy beam, an optical path provided with deviating mirrors or else an optical fibre allowing the laser beam to be conveyed to a welding head located opposite the pipe to be welded.
Conventionally, the welding head comprises a lens or one or more focusing mirrors so as to focus the laser beam onto one or more focal points in the thickness of the material to be welded and in the joint plane obtained by bringing together, edge to edge, the longitudinal edges of the metal sheet to be welded into the form of a tube (xe2x80x9cOxe2x80x9d forming), so as to locally concentrate sufficient power density to melt the material to be welded.
Usually, the welding head comprises a gas feed device for feeding with welding gas, also called assist gas, by means of a gas delivery nozzle placed coaxially with the laser beam. This gas feed device may also be external to the actual laser welding head.
An alternative solution for welding the edges to be joined together to form a tube consists in melting the said edges to be joined together in the joint plane by means of one or more electric arcs and using gas either as shielding gas or as active gas.
Such welding processes are also commonly used in industry; depending on the case, these are TIG (Tungsten Inert Gas), MIG (Metal Inert Gas), MAG (Metal Active Gas) processes or else plasma-arc or submerged-arc processes.
Such welding processes are described for example in the following documents: EP-A-847831, U.S. Pat. No. 4,673,121, EP-A-136276, JP-A-58148096, JP-A-03198998, JP-A-03198997, EP-A-896853, U.S. Pat. No. 5,192,016, U.S. Pat. No. 4,738,714, EP-A-899052, JP-A-58107294, EP-A-234623, U.S. Pat. No. 1,872,008, U.S. Pat. No. 4,396,20, U.S. Pat. No. 4,811,888 and U.S. Pat. No. 3,931,489.
However, laser welding processes or arc welding processes each have drawbacks which are specific to them.
In the case of the manufacture of a pipe, the process includes, in general, a phase of forming a pre-tube from a rectangular metal strip or sheet, followed by a phase of welding the pre-tube into a welded pipe.
According to a first technique, the welding is carried out axially, that is to say the metal strip is successively formed into a U and then into an O by bringing its two parallel longitudinal edges together so as to obtain an unwelded pre-tube, and then a longitudinal or axial welding of the two edges of the pre-tube to be butted is carried out with or without a groove in order to obtain an axially welded pipe, shown schematically in FIG. 1.
However, according to a second technique, the welding may be in a helix or spiral. In this case, the metal strip is firstly given a twist in a spiral movement so as to bring together or butt the two longitudinal edges of said strip in a joint plane having the form of a spiral or helix so as to form, here too, an unwelded pre-tube, this pre-tube then being subjected to helical welding so as to join the said two edges together in order to obtain a welded pipe.
Of course, in all cases the pre-tube and the welding head are driven so as to perform a movement of relative displacement one with respect to the other, that is to say either the pipe is stationary and the welding head moves, or vice versa.
The welding phase may be carried out in one or more passes and by using one or more welding processes according to the diameter and the thickness of the pre-tube to be welded.
In general, for thin-walled pipes, that is to say having a thickness of 1 mm to 6 mm, of small diameter, typically from 10 to 100 mm, the tube forming and welding operations are carried out almost simultaneously.
To do this, as mentioned above, a machine fitted with a line of press rollers, with well-defined profiles, progressively and continuously deforms the metal strip moving relative to the said rollers in order to give it the shape of the desired pre-tube, the two ends or longitudinal edges of the pre-tube then being welded together to obtain the pipe.
These operations are carried out at high speed and it is important to use welding processes which allow the desired penetration to be obtained without slowing down the forming process, that is to say also a desired minimum speed making it possible to maintain maximum productivity or, in any case, the highest possible productivity.
Industrial pipe manufacturing lines very often use the multicathode welding process which employs, in general, several plasma or TIG electric arcs aligned in the welding joint plane.
Sometimes, laser welding is also used to weld the pipes. In particular, compared with the multicathode process, the use of a laser makes it possible to increase the speed but to the detriment of increased precision, which therefore requires much more precise alignment of the edges to be welded and precise control of the gap between the edges to be welded. This is very expensive from the standpoint of the tooling to be used.
A process juxtaposing the above techniques is described in the document EP-A-496231 and consists in successively welding the edges of the pre-tube with firstly a laser beam and then with an arc delivered by a tungsten electrode. However, in this case, since the laser welding is carried out first, the problems similar to those mentioned above arise.
Furthermore, for thicker-walled pipes, for example having a thickness of 6 to 50 mm, the forming and welding operations are not in general simultaneous. The tube is firstly given its definitive form, that is to say its pre-tube form, and is then only welded.
For these thicknesses, the process is in general carried out in several passes, typically in three or more passes, namely a pass called seam-locking pass and two welding passes. To do this, the edges to be butted together are X-grooved and, approximately in the middle of the thickness of the joint, a heel is created which allows the seam-locking to be carried out by MIG welding, and which thus prevents any further deformation during the subsequent welding passes. Next, the tube thus seam-locked or prefastened is welded by means of a submerged-arc welding process on the back side and the front side of the joint, as shown in FIG. 3.
In view of this, one of the problems that still arises is to be able to have a welding process allowing effective welding of tubes of any thickness at high speed and/or with a limited number of passes, that is to say making it possible:
either to carry out the initial pass called the seam-locking pass at high speed (in the case of a tube with an X groove);
or to reduce the number of passes (2 instead of 3) by making a Y groove (cf. FIG. 2) instead of an X groove and then carrying out the initial seam-locking pass or the bottom pass at higher speed;
or, for thicknesses of between 6 and 10 mm, to carry out the welding in a single pass and without a groove.
It is an object of the present invention to solve the abovementioned problems by providing an improved pipe manufacturing process using a hybrid welding process which combines a laser beam with an electric arc.
The solution provided by the present invention is therefore a process for manufacturing a welded pipe from a metal strip having two longitudinal edges which are brought together so as to come approximately into contact with one another and thus form an unwelded pre-tube, the said pre-tube then being welded in order to join the said edges together, characterized in that the two longitudinal edges of the metal strip having a pre-tube form are welded together, in order to obtain a welded metal tube by using, approximately simultaneously, at least one laser beam and at least one electric arc, the said laser beam and the said electric arc combining at a single impingement point or impingement area in the joint plane to be welded so that the metal of the edges to be joined together undergoes localized melting and subsequent hardening into a welded joint which joins together the edges thus assembled.
Within the context of the invention, the longitudinal edges may be brought together:
so as to obtain a weld plane and/or joint which is approximately rectilinear or is helicoidal in shape;
either in a continuous manner, that is to say the edges are brought together gradually and locally as the metal sheet runs in and undergoes 0 forming and then welding; or in an overall manner, that is to say the two edges are brought together in their entirety and then welded (the case with thick-walled pipes).